T H E ABSORPTION SPECTRUM OF SUSPENSIONS OF CARBON BLACK A. J. WELLS
Department of Chemistry, Harvard L'niversity, Cambridge, Massachusetts AND
W. R. SMITH Godfrey L . Cabot, I n c . , Boston, Massachusetts Received February 28, 1941
Particle size and size distribution play an important r61e in determining the properties of finely divided pigments. A number of methods have been suggested for determining these two factors (4). Khile many of these Fethods are reliable for particles with average diameters larger than 1000 A . , they are of little value in evaluating rubberoreinforcing carbon blacks whose average part'icle diameter is around 300 A. Gamble and Barnett (A), extending the work of Pfund (lo), have suggested that the shape of the spect'ral transmission curve in the visible and near infrared for dilute suspensions of carbon black is a function of particle size and size distribution. I n the present paper we shall describe our attempts to duplicate the measurements of Gamble and Barnett with a nuniber of standard commercial carbon blacks. The absorption spectrum of carbon black suspensions has not been st,udied over any considerable range. Strong (12) measured the transmission of soot on a lacquer film for seven restrahlen wave lengths in the infrared. Gamble and Barnett (4) measured the spectrum of light transniit'ted by carbon black suspended in rubber cement over the wave length range from 0.4 to 4 p . Since the meane were available during the present investigation to extend these measurements, we hare studied the transniisnion of carbon black suspensions from 25 p in the infrared to 0.24 p in the ultraviolet. DESCRIPTIOS O F ESPERIJIEXTS
Preparation of samples The carbon blacks used in the investigation are known in the trade as P-33, Elf 25, and Xonarch 71. P-33 i, a non-reinforcing type of carbon black prepared by direct thermal decomposition of hydrocarbons. Elf 1055
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A. J. WELLS B S D IT-, R. SMITH
25 and Monarch 71 are channel-type carbon blacks prepared by the inipingement of natural gas flames on a cool surface. Elf 25 is typical of the rubber-reinforcing type carbons and also finds ext,enaive use in the ink industry. Monarch 71 R i widely used in the manufacture of paints and lacquers. The average diameters of these pigments (see table I) have been determined by the "dark field count" method (5) and have also been calculated from surface area measurements (2). In the latter case spherical particles of infinite uniformity were assumed in calculating the diameters. We feel that the surface area values are more reliable in the case of the tn-o channel blacks t'han those obtained by the count procedure. Forty-five parts of carbon black by iyeight were thoroughly dispersed in 100 parts of rubber on a laboratory roll mill. Ten grams of this stock were then diluted on the roll mill with 100 g. of rubber (smoked sheet). Ten grams of the diluted stock (2.8 per cent carbon black) were then sheeted thin on the roll mill and while still ivarm were cut up into 250 cc. of carbon tetrachloride, forming a thin cement. TABLE 1 Diameters of particles of three types of carbon blacks
I
SAMPLE
DIAMETER F R O M
DIAMETER F R O M
COUNT
BURFACEAREA
A.
P-33.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elf 2 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , / Monarch 7 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
...I
1570 696
550
A. ~
I
i
2200 320 100
A plate of an appropriate transmitting material (quartz, glass, rock salt, or potassium bromide) was dipped into this dilute cement from one end. After draining and drying, the plate was reversed and dipped again from the other end. After drying, the deposit was peeled off one side of the plate, leaving a film of reasonable reproducibility and fairly constant thickness on the other side. A blank was made by dipping a plate into a cement made from 10 g. of smoked sheet similarly milled, sheeted out, and dissolved in 250 cc. of carbon tetrachloride. Experiments in the infrared (6.5 to 26 p ) The infrared spectrometer described by Gershinowitz and Wilson (6) was used in this investigation. Potassium bromide (25 to 13 p ) , rock salt (13to 7.5 p ) , and fluorite (7.5t o 1 p ) prisms were used. This spectrometer is so constructed that a t each prism setting the reading through the sample is followed immediately by a reading through the blank. There fore the effect of the rubber cancels out in the calculation of the per cent
1057
ABSORPTION SPECTRUM OF CARBON BLACK
transmission, except in regions of high absorption by rubber. At these wave lengths small dips or peaks may occur, owing to slight inequalities in the thickness of the two rubber films. Potassium bromide and rock salt backing plates were used in this region. Figure 1 gives the transmission curve of Elf 25 over the whole range, together with curyes for Xonarch i l and P-33 in the potassium bromide region. These curves have been corrected for the effects of the rubber. The most interesting feature of the spectrum is the broad band centering a t 22.5 1.1.
Experiments in the near infrared, visible, and ultraviolef: 3 to 0.24 I.( The infrared spectrometer was used for measurement from 3 to 1 p . Because the light passes through the prism twice, there is the possibility of a relatively large amount of stray light in regions of low source energy.
c
I
i
, 2
,
4
,
6
,
1
8
;
8
,
8
l
1
8
1
,
12 14 16 I8 Wave Lenqfhin Microns
IO
I
l
20
i
l
22
l
f
24
26
FIG.1. Transmission curve of Elf 25 over the whole range, together with curves for Monarch 71 and P-33 i n the potassium bromide region. Curve A , P-33; curve B, Elf 25; curve C, Monarch 71. - - - -, Elf 25 on glass.
For this reason measurements to the blue of 1 p were found to be high, although it was possible to calibrate the instrument as far as 0.4 1.1. In the .risible and ultraviolet regions we used the spectrophotometer described by Jones ( 7 ) . Samples were backed on glass microscope slides in the near infrared and on quarts in the visible and ultraviolet. In fiyuro 2 are given curves (a, B, and C) for samples of the three carbon bi:;cks prepared as described above, extending to wave lengths as short as 0.4 p . The dashed portions of the curves indicate our estimate of the curves over the regions where no valid measurements xere made. The spectrum of P-33 was not determined on the spectrophotometer; the dashed line from 1 to 0.6 1.1 is an estimate based upon data taken from the infrared spectrometer. In order to carry the measurements into the ultraviolet, thinner films were necessary because of the limited intensity range of the spectrophotometer. Therefore films were made by dipping only once into the cement.
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A . J. WELLS AND W. R. SMITH
Curves B’ and C’ give the transmission of single thickness films of Elf 25 and Monarch 71 in the ultraviolet and near infrared, with an estimated connecting line through the visible. The striking result of these measurementb, as Gamble and Barnett (4) have shown, is the great drop in transmission as we go from longer to shorter wave lengths. A further effect, as yet unexplained, is the failure of the Elf 25 curve B in figure 2 to join smoothly onto that of figure 1. The only difference in the two cases is that of the backing,-in the first case glass, in the second case rock salt. The dotted line in figure 1 s h o w the glass-backed curve of Elf 25 continued into the infrared until absorption by the glass makes further measurements impossible. Around 3
70
-
60
-
-
(12
0.4
0.6
0.8
IO
1.2 1.4 1.6 1.8 2.0 2.2 Wave lengthin Microns
24
26
2.8
FIG.2. Transmission curves for samples of the three carbon blacks, extending t o wave lengths as short as 0.4w . Curve A , P-33; curve B, Elf 25; curve C, illonarch 71; curve B‘, single thickness of Elf 25; curve C’, single thickness of Monarch 71.
to 4 p there is a region of enhanced transmission, followed around 5 to 6 p by a region of absorption. Presumably this behavior is connected in some manner to the absorption of the glass in this region, although other results show that it does not depend upon differences in thickness of the plates. Similar experiments, not further reported here, were carried out on suspensions of carbon black in a high-grade varnish used in making inks. The results are in agreement with those given for suspensions in rubber. DISCUSSION OF THE RESULTS
Pfund (10) has shown that light scattered by a transparent pigment of uniform particle size follows the Rayleigh scattering law until the wave length is small compared to the particle size.
ABSORPTIOS SPECTRI-11 O F ChRBON BLACK
1059
Following Gamble and Barnett (J), we can write this law as
where I is the intensity of transmitted radiation, I o is the intensity of incident radiation, n and no are the refractive indices of the particle and medium, respectively, X is the nunher of particles, T7 is the volume of each particle, and X is the wave length of light. Assuming the refractive index terms to be constant, this formula can be w i t t e n :
Io d3 111 - = constant.I X4 where d is the diameter of the particle. Gamble and Barnett (4) attempted to use this formula to determine the particle size from measurements of the transmission spectrum. This can be done by noting the wave length a t which selective scattering ceases, or by fitting the experimental curve to a calculated Rayleigh curve. Their results are good in the case of zinc oxide and whiting, but do not agree very ne11 with our results on carbon black (see figure 7 of their paper). In no case were we able to reach a wave length short enough to give positive evidence of cessation of selective scattering, a not unexpected result, since, except for the case of P-33, the particle size should be well below the shortest wave length we used. It is also difficult to estimate particle size on the basis of the Rayleigh formula, because this equation does not describe the shape of the experimental curve. If the law holds, a plot of In I o / I against l / X 4 should be reasonably linear through the origin. Seither our data, nor those of Gamble and Barnett, are even approximately linear, nor do they go through the origin (this would amount to 100 per cent transmission at long wave lengths). Therefore we are inclined to view the method of Gamble and Barnett as unsatisfactory for carbon blacks of the channel type. While it may differentiate betweenoa non-reinforcing carbon of the P-33 type with a diameter of around 1500 d , it cannot distinguish between reinforcing carbons of the channel type whose mean diameter is around 300 A. The failure of the Rayleigh law is undoubtedly due t o selective absorption as well as scattering by the particles. The true transmission law should be written In
Io
=
d3 K(X) A4
+ A(X)
where K(X) is approximately constant but would express the variation due to refractive index and A @ ) is the function of the wave length rep-
1060
A . J. WELLS AND W. R . SMITH
resenting the actual absorption by the material. Unfortunately we have little information as to the shape of A(X). From the theory of metals (see, for example, reference 8) we know that, in general, large increases in reflectivity mean an increase in the absorption coefficient. Experiments on the reflectivity of polished carbon and graphite (1, 3, 11) show that in the infrared carbon begins to reflect as a metallic substance does in the visible. Such reflectivity in a metal is due to free electrons. In carbon with a graphitic structure it is due to the freedom of the resonance electrons among the many resonating benzenoid structures.’ Since the reflection shows a gradual monotonic increase from about 15 per cent in the visible to about 50 per cent a t 25 N , we would expect an increase in absorption coefficient in the same direction. We have been able to find no data on the reflectivity of carbon in the close neighborhood of our band a t 22.5 p , but it would be rather difficult to explain this band on any “metallic reflection” hypothesis. Likewise, the wave length is too large for the results to be affected by scattering. Instead we are inclined to think of the carbon black particle as a giant molecule which may, like other large molecules, show absorption in the infrared, owing to internuclear vibration. Presumably, with small particles a larger proportion of the normal modes of vibration would show a net change in the electric moment and hence be infrared active. Therefore, on an equal concentration basis the carbon black of small particle size should give the greater absorption. Our curves do show this order (see figure 1), but the differences are too small to be taken as significant without further experimentation. REFERENCES
(1) ASEHKINASS,E.:Ann. Physik [41 18, 373 (1905). (2) BRUSAUER, S.,EMMETT,P. H., IXD TELLER,E.: J. Am. Chem. SOC.60, 309 (1938). (3) COBLENTZ, W . : Natl. Bur. Standards (C. S.)Bull. No. 7, 197 (1911). (4) GAMBLE, D.C., A N D BARNETT, C. E . : Ind. Eng. Chem., Anal. Ed. 9,310 (1937). (5) GEHMAN, S.D.,A N D MORRIS,T. C.: Ind. Eng. Chem., Anal. Ed. 4,157(1932). (6) GERSHINOWITZ, H., A K D KILSOX, E . B., J R . : J . Chem. Phys. 6, 197 (1938). (7) JONES, R . S , : J. ilm. Chem. Sac. 62,148 (1940). (8) MOTT,N.F., AND JONES, H.: The T h e o r y of the Properties of Metals and Alloys, p . 105. University Press, Oxford (1936). (9) PAULIKG, L.:The Nature of the Chemzcal Bond, p. 172. Cornel1 University Press, Ithaca, Kew York (1940). (10) PFUND,A. H.:J. Optical Sac. Am. 24, 143 (1934). (11)SENFTLEBEN, H.,AXD BENEDICT, E.: Ann. Physik 64,65 (1917). (12) STRONG, J.: Phys. Rev. 37, 1565 (1931). 1
See reference 9 on resonance in graphite.
